Thermal contact resistance or, its reciprocal, thermal contact conductance is an important parameter in a wide range of thermal phenomena. It plays a significant role in heat transfer applications such as electronic packaging and nuclear reactors. This parameter also appears in fin-tube heat exchangers; however, it is often neglected in the performance calculations of heat exchangers. This thesis project explores the means by which the heat transfer performance of a finned tube heat exchanger may be enhanced. It includes experimental studies and finite element analysis investigating the effects of expansion bullets and coatings on the thermal contact conductance. An apparatus has been designed and fabricated for the experimental part of the work. A finite element model established the fintube configuration to be used in the design and manufacture of the apparatus. The apparatus was specially made for measuring thermal contact conductance directly in a finned-tube heat exchanger both in vacuum and in gaseous environment. The experiments were done on hexagon shaped specimens with a single fin connecting seven tubes. Sixteen type-T thermocouples have been used to measure temperatures at several locations on the specimen. A full-scale quarter-fin model was chosen for a second finite element analysis. The model simulates the actual specimen and predicts the temperatures. The finite element analyses have been used to validate the experimental results. The experimental results from the bare contact specimens, assembled with different sizes of expansion bullet, show that while higher expansions enhance the thermal contact conductance, the effect of interstitial gas such as nitrogen is beneficial for all specimens expanded with the 9.42 mm size bullet. Applying a coating material with high thermal conductivity is also an effective way to enhance the thermal contact conductance. The results show that the highly conductive plating materials, such as zinc, tin, silver and gold, enhance the thermal contact conductance. The presence of interstitial gas such as nitrogen also results in higher heat transfer rates and higher thermal contact conductance compared to those obtained in vacuum.
|Creators||Cheng, Wui-wai, Mechanical & Manufacturing Engineering, Faculty of Engineering, UNSW|
|Publisher||Awarded by:University of New South Wales. School of Mechanical and Manufacturing Engineering|
|Source Sets||Australiasian Digital Theses Program|
|Rights||Copyright Wui-wai Cheng, http://unsworks.unsw.edu.au/copyright|
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